This invention relates to dc-to-dc converters, and more particularly to improvements over the new optimum topology switching converters disclosed in an application Ser. No. 837,532 by Slobodan M. Cuk and Robert D. Middlebrook filed on Sept. 28, 1977.
In the new switching converters disclosed in the aforesaid application, two inductances are employed, one in series with the input source and the other in series with the output load, together with a storage capacitance between the inductors. Suitable switching means are provided for alternately connecting the junction between the input inductance and the storage capacitance, and the junction between the output inductance and storage capacitance, to return current paths for the source and load. The result is that dc-to-dc conversion is achieved with both input and output (inductor) currents non-pulsating, i.e., with only a small switching ripple inversely proportional to the inductance values superimposed on their dc (average) values.
Simplification of this new switching converter topology and further improvement of its performance is obtained by the technique of coupling the inductors of this converter into a single magnetic circuit with two windings as described in detail in the aforesaid patent application. The coupling of inductances leads to at least a reduction to half of both current ripples. However, by proper design it is possible to obtain zero-current ripple on the output (or the input), while the current ripple at the input (or the output) remains at its unreduced level, i.e. its level before the coupling. The coupled inductances thus provide nonpulsating currents with ripple reduced to zero at either the input or the output, or reduced by one half at the input and the output.
In applications where the source and the load may not be referenced to a common ground, an isolating transformer may be inserted by dividing the energy transferring storage capacitor into two series capacitances, and then connecting one of these capacitances in series with the primary winding of the isolation transformer, and the other capacitance in series with the secondary winding of the isolation transformer. The input and output inductances may still be coupled to reduce the input or output current ripple to zero, but not both. As in the case of the nonisolated converter with coupled inductances, either a balanced reduction of ripple on both sides, or an imbalanced reduction of ripple on either side can be achieved by a proper design (or adjustment) of the coupling.
However, the coupled-inductor version of either the isolated or nonisolated extension of the optimum topology converter provided, by a proper design, zero current ripple at only one side, either the converter input or output side, but not at both simultaneously. This then naturally led to a search for such a switching converter configuration which will achieve these ideally desired characteristics of zero current ripple at both input and output simultaneously. With a high efficiency constraint in mind, the further objective was to achieve these goals with the least number of storage elements and switches put together in the most favorable topology, hence the optimum topology converter described in the copending application was used as a basis.
From another viewpoint, the coupled-inductor extension of the optimum topology converter (both isolated and nonisolated versions) was the first time that two magnetic components (inductors), which are normally and exclusively used separately to perform their function, have been integrated into a single magnetic circuit (single core) with two windings, resulting in size and weight reduction, component count reduction and performance improvement. This then motivated the search for such switching configurations in which the integration of otherwise independent and separate magnetic components can be achieved at an even higher level, by incorporating both separate inductors and AC transformers into a single magnetic circuit with additional size and weight reduction as well as performance improvement.
Both of these objectives, that is zero ripple at both input and output as well as magnetic circuit simplification and reduction of its size and weight, have been achieved in the switching converter configuration as disclosed by this invention. A number of improvements over the previously disclosed optimum topology converter (Application Ser. No. 837,532) are demonstrated. In addition, it is shown how some other known as well as new switching converter configurations may be improved by the integrated magnetic circuit technique disclosed in this application.